Mobile networks are currently evolving from pure circuit switched (CS) networks towards internet protocol (IP) based packet switched (PS) networks, and by that seamlessly integrate into IP based infrastructure that is also used for the Internet, the World Wide Web and the datacom industry. One trend seen on the telecommunications market is that mobile operators want to migrate their customer base from the CS domain towards IP multimedia subsystem (IMS) domain in order to open up for new revenue streams from the new services available in IMS. Another trend to be observed is that mobile operators try to increase the revenues by extending the customer base to the fixed access domain. This is done by acquiring new fixed subscribers, but also by offering a fixed line access for existing mobile subscribers.
An essential problem of IMS migration is that customers are used to the services that are available in the CS domain. Those services include the supplementary services as defined for global system for mobile communications (GSM), but also operator specific services and customized services implemented via the capabilities of intelligent networks (IN) and customised applications for mobile networks enhanced logic (CAMEL).
In order to resolve the above problem, the 3rd Generation Partnership Project (3GPP) has specified the IP Multimedia Service Switching Function (IM-SSF). The IM-SSF is, as defined in the corresponding 3GPP specification TS 23.218, a CAMEL functional entity that provides the interworking between session initiation protocol (SIP) session control and the CAMEL state models. The IM-SSF also provides a CAMEL interface to the home subscriber server (HSS) for downloading the subscriber's CAMEL Subscription Information (CSI) data for IMS. In fact, the IM-SSF is a session initiation protocol (SIP) application server (SIP-AS) that interfaces SIP to a CAMEL application part (CAP) similar signalling protocol.
The IM-SSF is a SIP Application Server (SIP-AS) in IMS, enabling provisioning of CS services to users of the IMS network. It therefore has an incoming SIP signalling data stream, an outgoing SIP signalling data stream, and at least an outgoing CAP-like data stream. The CAP-like data stream provides detection points to a sort of CAMEL service environment (CSE), enabling provision of the desired CS services.
Even though the CAP-like data stream provided by the IM-SSF corresponds to a 3GPP standardized CAP protocol version, it is tailor made for the IM-SSF and therefore slightly differs from CAP versions as used in a standard CSE in an IN/CAMEL network. The reason for using a tailor made CAP version with the IM-SSF is that the IM-SSF uses IP multimedia basic call state model (IM-BCSM) as the basic call state model (BCSM) on the basis of which detection points (DP) in CAP are provided to the CSE. IM-BSCM is the BSCM that is used for SIP signalling in the IMS network, and due to the different nature of IMS networks and IN/CAMEL networks, it differs from basic call state models used in IN/CAMEL networks, such as O-BCSM or T-BCSM under standard CAP versions used in circuit switched networks. In order to provide services based on this tailor made CAP-like protocol used by the IM-SSF, it requires a CSE which is tailored to the CAP-like data stream provided by the IM-SSF. A standard CSE available in regular IN/CAMEL CS networks cannot be applied, because the CAP version of the CAP data stream supported by the IM-SSF cannot be processed in such an environment. The tailor made CSE used for providing the services to users of the IMS network is therefore an adapted version of a regular CSE, and is made available in the IMS network where it is usually implemented.
Due to the mismatch between IM-BCSM based detection points provided by the CAP date stream from the IM-SSF, the adapted CSE used by the IM-SSF for providing services in the IMS is a restricted environment wherein a limited number of services may be provided. Each new service to be provided in the IMS network has to be implemented separately in the CSE tailored for the IM-SSF. This re-implementation of services in IMS (which services were already available in the CS network) is costly, as it has to be tailored to the non-standard CAP-like version provided by the IM-SSF.
On the other hand, existing implementations of the services in the circuit switched network, subject to main investments at the time these implementations were made, are left unused and thus gradually become obsolete.